They’re not meant to be niche languages. They’re meant to be general purpose languages. They’re trying to use functional programming to achieve a high degree of correctness with short, readable programs.
Compilers are a good one. I know Rust's original compiler (before being self hosted) was implemented in OCaml. Darklang's compiler was in OCaml as well.
Not niche, Jane Street uses OCaml (and they contribute a lot to the compiler, too), it is "a research-driven trading firm where curious people work together on deep problems".
> where curious people work together on deep problems
'curious people': people who got jaded by academia and were attracted by the six- to seven-figure salaries at Jane Street.
'deep problems': Buy X units of Y instrument at A exchange, and sell Z units of said instrument at B exchange, and do this often enough that said company makes a pile of money for itself and its employees (mostly itself, given it can afford to pay its employees six to seven figures).
